I agree, it is prescriptive, but most people never get beyond (John 3:16). John explains what he means by the verse, in the verses which follow....which no one ever seems to Quote.
What does it mean to believe in Jesus Christ?
Many people only regard Jesus as the Son of God. That's great, but Jesus is also....the Lamb of God. And this is the focus of (John 3:16).
If you believe Jesus is the Lamb of God, how will you show you believe in him?
By using his sacrifice through the confession of your sins...
If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. (1 John 1:9)
If you don't confess your sins, it shows God that you do not believe Jesus is the Son of God....and furthermore, that you are not a sinner in need of forgiveness,....If we claim to be without sin, we deceive ourselves and the truth is not in us. If we claim we have not sinned, we make him out to be a liar and his word is not in us. (1 John 1:8,10)
Now look at the rest of (John 3) along with (1 John 1). And you can see there, he gives a clearer explanation of what he meant by (John 3:16). There is so much more to believing Jesus is the Son of God, than the mental exercise taught by many churches. But again...don't take my word for it, confess your sin before God and see how God reacts. If there is no change...I am a Liar. But if there is a reaction after you confess sin, I am not a Liar..but one who tells you the Truth.
How does this prove my point?
If confession of sin is for the forgiveness of sin, and you feel that forgiveness after the confession, this proves your sins were not forgiven.
If your sins were not forgiven, how can you believe you are saved?
To be saved, your sins must be forgiven...